ldap-sync: fix LDAP_CA_CERT_PATH default to system CA bundle
The Bitu LDAP server (ldap-ro.{eqiad,codfw}.wikimedia.org) uses a Let's Encrypt cert, not the WMF internal CA. The system CA bundle ships LE roots; the WMF CA file does not. Without an explicit LDAP_CA_CERT_PATH override, the sync would TLS-fail at the LDAPS handshake on first run.
Default LDAP_CA_CERT_PATH to /etc/ssl/certs/ca-certificates.crt, bringing it in line with SPEC.md.
Bug: T420691